3. generator circuit breaker
TRANSCRIPT
CHARLES ISIADINSO
GENERATOR CIRCUIT BREAKER
(GCB)
A circuit breaker is an electronic switch designed to isolate a circuit
in the case of overload or short circuit. The GCB typically installed
between the generator and the main generator step-up (GSU)
transformer, electricity leaves the generator and enters the
generator circuit breaker via isolated-phase bus (device used to
conduct electricity because of the magnitude of the power being
transmitted). Its purpose is to create a breakpoint in the system and
is typically used to protect the overall system in the event of
overload or any other electrical fault.
The GCB works by automatically interrupting current flow at the
onset of a fault. GCBs have two contact points, a physical
connection (made up of two fixed and mobile units) and a gaseous
connection, which is usually sulfur hexafluoride (SF6) gas because
it is twice as resistant to the flow of electricity as air. Power stations
usually transfer electricity at very high voltages (between 72000V
and 1200000V), at these voltages if contacts are simply separated;
an electrical arc forms bridging the gap created and continuing the
flow of electricity. This is counterproductive as the main point of
breaking the connection is to stop flow of electricity. To solve this
problem, the temperature conditions and distance between the
contacts has to be controlled. To achieve effective control of the
arc, the separation takes place in an arc chamber filled with SF6
gas. The physical connection is designed with two parts, a
permanent contact (located close to the chamber walls) and arc
contact (located at the center of the chamber and specifically
designed to withstand electrical arcs); the gaps between the
contacts are also filled with SF6 gas.
To carry out a break, the permanent contacts are separated leaving
only the arc contacts. This concentrates the flow of electricity in
the arc contacts. The arc contacts are then separated and a
controlled electrical arc is formed between the arc contacts. The
CHARLES ISIADINSO
electrical arc causes rapid rise in the temperature and pressure of
the SF6 gas. The SF6 molecules explode and lose their insulating
qualities. The rise in pressure causes high-pressure gas from the
center of the chamber to move to the low-pressure part of the
chamber (usually the back), the gas moves taking the arc with it,
and this extinguishes the arc and completes the circuit break. The
SF6 quickly cools, recombines, and regains it insulating capacity.
The entire break is completed in 50 milliseconds.
REFERENCE:
1. "Generator Circuit-breakers (GCB)." Generator Circuit
Breakers. ABB., n.d. Web. [Accessed17 June 2014].
2. "Circuit Breaker." Wikipedia. Wikimedia Foundation, 06 July
2014. Web. [Accessed 17 June 2014].